At least seven separate incidents involving grenades have been identified in the last two years, according to the San Juan Police Department. In one incident, a hollowed-out grenade, seen above, was stuffed with a death threat before being thrown into the window of a home. (Courtesy: San Juan Police Department)

Mexican drug cartels, whose trafficking efforts into the U.S. have grown bolder amid the border crisis, are increasingly utilizing a frightening weapon in their arsenal: the hand grenade.

Whether packed with explosives, hollowed out and filled with steel to give them authentic heft or even used to deliver handwritten death threats, grenades are turning up in greater frequency in encounters with suspected cartel members, according to law enforcement authorities and border experts. U.S. Immigration and Customs Enforcement officials confirmed to FoxNews.com that they’ve seen a “trend increase” involving the devices along the entire southwest border, although details were not disclosed due to ongoing investigations.

“The reason you’re seeing so many more [grenades] this year is because much more heavily-armed drug shipments are coming into the United States,” said James Phelps, an assistant professor in the Department of Security Studies and Criminal Justice at Angelo State University in Texas. “With Border Patrol so heavily distracted doing paperwork and watching the mass flood of people coming into the country, they don’t have as much time to do what they used to do — drug interdiction.”

“The reason you’re seeing so many more [grenades] this year is because much more heavily-armed drug shipments are coming into the United States.”

– James Phelps, assistant professor, Angelo State University

As a result, in Nuevo Laredo, Mexico, just across the Rio Grande from Laredo, Texas, bystanders can hear a grenade or two “go off every night,” Phelps told FoxNews.com. The devices, which sell for up to $500 on the Mexican black market, are primarily surplus military grenades obtained by cartels in Central America, where weapons from the civil wars in El Salvador and Nicaragua remain readily available.

“The cartels buy them up in mass when they’re found, just like they do with the rest of their guns and artillery,” Phelps continued. “We’ve seen it all across Mexico and we’ve seen it in Central America. It’s not going to be long until we see it pop up in our country.”

Gillian Christensen, a spokeswoman for U.S. Immigration and Customs Enforcement, confirmed to FoxNews.com that federal officials have seen a “trend increase” involving grenades along the southwest border. Additional details, however, cannot be disclosed due to ongoing investigations, she said.

In rural Starr County, Texas, five grenades were found on Aug. 3 at a home near Rio Grande City where two Honduran nationals were murdered. A third man was also badly wounded and later died at a local hospital. Starr County Sheriff’s Office Capt. Carlos Delgado told FoxNews.com the grenades were initially thought to be live but were later determined to be inert. He declined to speculate as to where the devices came from or to confirm that the incident is believed to be cartel-related.

“It’s going a lot further than a triple homicide, but we do not have a suspect in custody,” Delgado said of the ongoing investigation.

Delgado said heavily-armed cartels primarily utilize grenades as intimidation in local communities.

“It would be power,” Delgado said of motivating factors. “You would definitely not attack me if I have grenades. It empowers.”

In nearby Hidalgo County, one of the fastest growing counties in the country, at least seven separate incidents have involved grenades within the last two years. San Juan Police Chief Juan Gonzales, who did not return several messages seeking comment, told The Monitor that cartels and local gangs use grenades to boldly intimidate others. Those manufacturing them are likely not inexperienced criminals, but rather former Mexican military personnel familiar with explosives, he said.

“They purchases the parts and build them here,” Gonzales told the newspaper.

In one incident, a hollowed-out grenade was stuffed with a death threat before being thrown into the window of a home, he said.

“It’s one of those things where they are trying to instill dear into their rivals and law enforcement,” Gonzales told the newspaper.

An incident in late 2009 — when a man was arrested after he sold nearly 200 grenades to an undercover agent posing as a drug cartel — was the first time the department realized explosive devices could become a serious threat.

More recently, in late 2011, U.S. Customs and Border Patrol agents reportedly found one grenade launcher and a rocket launcher in a bag buried in the brush near the Rio Grande in Fronton, Texas.

Finally, according to an FBI intelligence bulletin released in 2011, the Los Zetas cartel – infamous for brazen kidnappings-for-ransom and sheer brutality – began recruiting and relying on “non-traditional, non-military trained associates,” or US-based prison and street gangs or non-Mexican nationals to support its drug trafficking operations. The gang had begun stockpiling weapons, including assault rifles and grenades, at safe houses in the United States years earlier in response to crackdowns on both sides of the border against drug traffickers.

Jaime Gonzalez Duran, one of the organization’s original founding members, reportedly ordered Zetas near McAllen, Texas, to regain control and engage law enforcement officers if confronted. When “El Hummer” was arrested in late 2008 by Mexican Federal Police and the Mexican Army, officials took custody of what was then the largest weapons seizure in the country’s history, including 540 rifles, nearly 300 hand grenades and sticks of dynamite. He was later sentenced to 35 years in prison in 2012.

Phelps, meanwhile, said he expects to see an increase of incidents involving grenades along the border moving forward as cartels have become more brazen about potential gun battles with law enforcement.

“At some point, probably within the next 6-12 months, you’re going to start seeing shooting incidents between the cartels and the agencies attempting to stop them as they move further into the United States,” he told FoxNews.com.

While lawmen will likely not submit it on-record, Phelps said the worsening situation has already injected fear into some officers.

“If you see a nice Ford F-150 with a covered bed on it and heavily-tinted windows and you’re down in Laredo, Texas, and there’s a ‘Z’ on the back window or a Santa Muerte, you don’t pull them over,” he said. “You’re smarter than that, to pull them over. Even if they blatantly violate the law, you let them go because you don’t know who you’re walking up on. And the closer you get to the border, the worse it is.”